The increasing popularity of portable consumer electronic products such as mobile phones, MP3 players and PDAs has prompted new concerns on the huge variety and number of battery chargers that are required and which are costly, inconvenient and eventually lead to electronic waste problems. Inductive or wireless charging apparatus that can charge more than one electronic product have been proposed. Two different approaches to the generation of AC magnetic flux have been proposed, namely “horizontal flux” and “vertical flux” methods.
Inductive electronic chargers with a direct connection have been developed for use with some types of portable electronic equipment such as electric toothbrushes. Inductive chargers have also been proposed in a number of documents such as U.S. Pat. No. 6,356,049, U.S. Pat. No. 6,301,128, U.S. Pat. No. 6,118,249. These inductive chargers, however, use traditional transformer designs with windings wound around ferrite magnetic cores and the main magnetic flux between the primary winding and secondary winding has to go through the magnetic core materials. Other contactless chargers proposed (e.g., Chang-Gyun Kim; Dong-Hyun Seo; Jung-Sik You; Jong-Hu Park; Cho, B. H., “Design of a contactless battery charger for cellular phone,” IEEE Transactions on Industrial Electronics, Volume: 48, Issue: 6, December 2001 Page(s): 1238-1247) also use magnetic cores as the main structure for the coupled transformer windings. However, these battery chargers do not use a planar structure and each charger is only able to charge one item of electronic equipment at a time.
Recent research in the field of planar magnetics and planar transformer technology has prompted the development of planar contactless battery charging systems for portable electronic equipment. Among them, two proposals are particularly of interest, because they allow one or more items of electronic equipment to be placed and charged simultaneously on the charging surface, regardless of the orientation of the electronic equipment.
The first type of planar battery charger modifies the rotating machine concept by flattening the “round shape” of the motor into a “pancake shape,” as described in GB2399225A, GB2398176A, WO2004/038888A, GB2388716A, US2003-210106-A1, GB2392024A, and GB2399230A. The magnetic flux lines flow horizontally along (roughly in parallel to) the planar charging surfaces. The portable electronic equipment to be charged by the charging device needs a secondary winding wound on preferably a soft magnetic core. An AC voltage will be induced in this secondary winding for charging the battery, usually via a battery charging circuit.
A fundamental and inherent limitation of this type of battery charger is that this charging device must have a good electromagnetic flux guide to confine the flux along the lower surface. Otherwise, if such a charging device is placed on a metallic table or a conductive surface, induced current will circulate in the metallic table or conductive surface, resulting in heat generation and power loss in the metallic table or conductive surface. One imperfect way to solve this problem is to place a piece of soft-magnetic material (such as a layer of ferrite, iron power or amorphous soft magnetic alloy) as a magnetic flux guide under the lower surface. However, if the electromagnetic flux is large, a fairly thick layer of soft-magnetic material is needed, defeating the purpose of designing a “thin” charging platform and increasing the cost due to the large amount of soft magnetic material required. In addition, the electromagnetic shielding effect of using one layer of soft magnetic material may not be sufficient for electromagnetic compatibility (EMC) requirements. Some flux may still penetrate through the soft magnetic layer and induce current in any conductive surface below the charging platform.
A better solution to shield the magnetic field in the lower surface is to use a combination of a layer of soft magnetic material and a conductive material as disclosed in US2003-095027-A1. It is important to note that the addition of a thin layer of conductive material can significantly increase the shielding effectiveness as reported in US-2003-095027-A1, U.S. Pat. No. 6,501,364, and Tang S. C., Hui S. Y. R and Chung H., “Evaluation of the Shielding Effects on Printed-Circuit-Board Transformers using Ferrite Plates and Copper Sheets,” IEEE Transactions on Power Electronics, Vol. 17, No. 6, November 2002, pp. 1080-1088.
The second approach described in WO03/105308A, GB2389720A, GB2399446A, GB2389767A, GB2389767A, WO2007/019806 is to create an AC magnetic field with the flux lines flowing substantially vertically out of the planar charging surfaces, i.e., in a direction substantially perpendicular to the plane of the charging platform. Since the lines of flux enter and leave the planar charging surface vertically, a very thin secondary coil can be used to pick up the magnetic flux. This results in the possibility of a slim design for the secondary module that can be embedded in the portable electronic load.